This invention relates in general to equipment for processing natural gas to remove heavy hydrocarbons, and in particular to a turboexpander compressor.
A turboexpander compressor is an apparatus used in a natural gas processing plant for lowering the temperature and pressure of incoming rich or process gas. The turboexpander compressor has a shaft mounted in a housing on bearings. An expander turbine wheel is located in an expander chamber on one end of the housing and a compressor wheel is located in a compressor chamber on the other end of the housing. High pressure process gas spins the expander wheel, thereby lowering the pressure and the temperature of the gas. The low temperature gas then proceeds to separators where the heavy hydrocarbon components are separated out. The turboexpander compressor is used in two different types of systems. In the post booster system, the warm, dry gas after separation proceeds to the compressor, which compresses the gas up to a sales line pressure or an intermediate pressure. The compressor is driven by the expander. In the prebooster type of system, the process gas from the incoming line is compressed prior to the gas proceeding to the expander.
The shaft spins typically at 25,000 to 60,000 rpm (rotations per minute). The gas pressure at the expander inlet typically is 600 to 800 psi (pounds per square inch). The temperature leaving the expander is typically minus 140.degree. F. (degrees Fahrenheit). Consequently, the seal system must seal a shaft at relatively high pressures at high rotational speeds and at low temperatures.
The shaft is supported normally on fluid film type bearings. An oil pump circulates a high flow rate of oil through the bearings. Labyrinth seals are located at the ends of the housings, sealing the shaft ends from the expander and compressor chambers. These labyrinth seals comprise annular grooves closely spaced to the housing bore but not in contact.
While labyrinth seals are durable and do not expend energy, they have a relatively high rate of leakage. If the process gas is allowed to leak through the labyrinth seals and mix with the lubricating oil, the oil will be diluted because of the heavy hydrocarbons present in the process gas. This lowers the viscosity of the oil and reduces its effectiveness. Also, cold gas from the expander leaking into the bearings will freeze the oil and thus damage the system.
Many systems employ seal gas to prevent leakage of process gas from the expander and compressor chambers into the bearing areas. One such system is shown in U.S. Pat. No. 3,420,434 issued Jan. 7, 1969 to J. S. Swearingen. Seal gas systems in general have means for injecting a warm, dry, filtered and lean gas around the shaft between the labyrinth seals and the bearings. This gas is at a pressure sufficient to avoid process gas from leaking past the seal gas into the bearings. The warm seal gas also serves as a thermal barrier to prevent the cold temperatures from freezing the oil.
Although the above-mentioned patent purports to employ seal gas from the compressor discharge, in general, seal gas must come from an exterior supply. One reason is that in the prebooster system, the gas which is being discharged from the compressor would still be rich in hydrocarbons and would dilute the lubricating oil. In a post booster system, the gas at the compressor discharge is warm and lean. However, often there is insufficient pressure developed across the compressor to provide a seal gas pressure sufficient to prevent leakage of oil into the compressor chamber. The oil is pumped at a high velocity, the velocity creating a pressure head that is too high for the seal gas to completely stop. Leakage of oil into the compressor and expander chambers can result in damage to the compressor and expander wheels. Also, if the system is shut down, oil will leak into the compressor and expander chambers because there will be no seal gas pressure.